Definitely Debussy. I'm probably going to get flamed for this, but I just can't stand most of Ravel's music. I don't care if you think I just don't have a mature ear (or whatever else people say because I don't like him), but most of his songs are just random notes.
Debussy's music, on the other hand, is so beautiful it's unreal. There's nothing else like it. My favorite pieces are Clair de Lune, Prelude, and Passipied from the Suite Bergamasque, Arabesque 1, and Sarabande from Pour le piano.